As Above so Below? - Impacts of Water Limitation on Growth and Nutrient Accumulation of a Crop Plant and its Soil Microbes Across a Fertility Gradient
Strategies to increase crop yields and drought stress tolerance are urgently needed to meet future global food demands in a changing climate. Although the effects of fertilizer application and drought stress in agriculture have each been researched extensively, the combined impacts of these two factors on crop yield, soil microbes, and nutrient availability are not well understood. High soil nitrogen availability promotes rapid plant growth, and increases leaf sap nitrate concentrations, but reduces carbon-based solute concentrations. By contrast, plants accumulate carbon-based solutes in response to drought. This project evaluates changes in leaf sap composition in response to fertilizer and water availability, to determine whether a trade-off exists between reduced leaf sap carbon concentrations associated with rapid plant growth, and subsequent tolerance to water-limitation. My thesis research centred on a greenhouse experiment to investigate the effects of nitrogen and potassium fertilizer (NK) additions at three levels on soil microbes, shoot growth, and leaf sap composition of replicate (n=9) Collard greens (Brassica oleracea) before and after a moderate water-limitation treatment. Increasing levels of NK addition progressively enhanced shoot biomass, and the medium and high levels doubled leaf sap total nitrogen and halved leaf sap carbon concentrations after 63 days of growth (i.e. at the end of the ‘normal’ trial). However, after three-weeks of subsequent water-stressed conditions (i.e. at the end of the normal plus water-limitation phases – the ‘water limitation’ trial), shoot biomass increases maintained a similar growth pattern to the normal trial, indicating that low leaf sap carbon concentrations did not reduce the subsequent shoot growth rate under water-limited conditions. Soil microbial N accumulation was equivalent across all fertilizer addition levels at the end of the normal trial, but increased progressively with NK addition after the water-limitation trial. Finally, leaf sap nitrate correlated closely with soluble soil nitrate, indicating that the former may provide an effective low-cost in-field test to determine the status of soluble nitrogen in agricultural soils. Overall, I found new and important relationships among leaf sap, soil microbial biomass, and bulk soil nutrient pools that will contribute to the improvement of soil management practices to meet the agricultural challenges of the future.